The present invention relates to a Brillouin/Erbium laser arrangement and in particular, to a bidirectional single elliptical polarization high efficiency and output power all fibre laser arrangement and its application to current sensing.
Brillouin fibre lasers (BFL) are particularly useful in fibre sensors due to extremely narrow linewidths and they have been the subject of considerable research at a number of different wavelengths [S. P. Smith, F. Zarinetchi, and S. Ezekiel, Opt. Lett., vol. 16, p. 393,1991]. They have found applications such as gyroscopes [S. Huang, K. Toyama, B. Y. Kim, and H. J. Shaw, Opt, Lett., vol. 18, pp. 555-557, 1993] and current sensors [A. Kxc3xcng, P. -A. Nicati, and P. A. Robert, IEEE Photon. Technol, Lett., Vol. 8, p. 1680, 1996]. Such devices are realized by using two counter-propagating Brillouin lasers sharing the same fibre ring resonator and experiencing different apparent cavity lengths due to nonreciprocal phaseshifts induced by either Sagnac or Faraday effects. These two counter-propagating Brillouin lasers will therefore oscillate at slightly different optical frequencies and the optical beat frequency will be linearly proportional to either the rotation rate or the electric current. These types of sensors have much simpler signal processing than more conventional devices [I. G. Clarke, Opt. Lett., vol. 18, pp. 158-160, 1993] based on Sagnac loop. To achieve maximum sensitivity of the current sensor to the Faraday effect it is necessary to keep the laser polarization states circular although some degree of ellipticity is acceptable.
The conventional technique to produce a BFL is to construct a critically coupled fibre resonator, required because of the small magnitude of the Brillouin gain [G. P. Agrawal, in Nonlinear Fibre Optics, Academic Press, San Diego, Calif., 1989]. Principal disadvantages of critically coupled BFLs include the small output power that can be achieved, the requirement of cavity matching to the pump signal, and the difficulty in incorporating intra-cavity elements because of their associated loss.
Brillouin/erbium fibre lasers (BEFL) [G. J. Cowle and D. Yu. Stepanov, J. Lightwave Technol., VOL. 15, PP. 1198-1204, 1997] combine Brillouin gain as used in BFLs with gain from erbium-doped fibre (EDF) to yield laser sources with unique properties. Narrow bandwidth nonlinear gain from stimulated Brillouin scattering in Single Mode Optical Fibre (SMOF) precisely determines the wavelength of operation, and gain in erbium-doped fibre (EDF) allows efficient operation and large power extraction. BEFLs do not require cavity matching to the Brillouin pump frequency since the BEFL resonator is not critically coupled. The Brillouin pump can be removed from the laser cavity to avoid spurious injection locking. In [G. J. Cowle and D. Yu. Stepanov, Opt. Lett., vol. 21, pp. 1250-1252, 1996] the laser cavity was made non-resonant in the direction of the Brillouin pump injection using a pigtailed bulk isolator. However, such an approach is undesirable for the sensing applications which require bidirectional operation.
It is an object of at least preferred embodiments of the present invention to provide an improved laser arrangement which overcomes the disadvantages of the aforementioned laser arrangements and is further preferably capable of being applied to electric current measurements.
In accordance with a first aspect of the present invention there is provided a laser arrangement comprising a ring cavity, at least two gain portions in said ring cavity, an input-output coupling connected to said ring cavity, a discriminator means connected in said ring cavity adapted to discriminate between a pumping signal and an output signal, and pumping means for providing said pumping signal and connected to said gain portions of said ring cavity such that, upon activation of said pumping means, said laser arrangement operates as a bidirectional laser producing a laser output at said input-output coupling.
In one embodiment, the laser arrangement comprises a ring cavity with Brillouin gain portions, a bidirectional optical amplifier and an elliptical polarizer interposed in predetermined portions of the ring cavity, input-output couplings connected to the ring cavity, and pumping means connected to the Brillouin gain portions of the ring cavity, such that, upon activation of the pumping means, the laser arrangement operates as a bidirectional laser and produces an output at said input-output couplings.
Preferably the laser arrangement acts as a Brillouin laser for obtaining a bidirectional single polarization high power output at the frequencies shifted down from the frequencies of the pumping means by an amount determined by the Brillouin shift in the glass fibre.
In accordance with the second aspect of the present invention the aforementioned laser arrangement is applied to the measurement of an electric current by measuring the optical beat frequency between counterpropagating waves in the ring cavity. The optical beat frequency being linearly proportional to the current value.
Preferably, the optical amplifier is formed from the one piece of rare earth doped optical fibre with a separate pumping means connected to the rare earth doped optical fibre.
Preferably, the elliptical polarizer is formed from the one piece of spun highly birefringent polarizing fibre (PSHBF) polarizing in the wavelength range of the laser operation [I. G. Clarke, Opt. Lett., vol. 18, pp 158-160, 1993].
The Brillouin pump should be removed from the ring cavity to enable the laser to be operated in the BEFL mode. This can be achieved using a number of different techniques with the preferred embodiment utilising a polarizing spun highly birefringent fibre. Alternatively, a blazed grating or other discriminators, could be utilized, provided that the discriminator does not inhibit the bidirectional operation of the laser arrangement.
In broad terms, the preferred embodiment of the present invention provides a Brillouin laser, comprising Brillouin gain portions and a rare-earth doped optical amplifier inserted into a ring cavity.
More specifically, the preferred embodiment of the present invention provides a bidirectional single polarization high output power laser arrangement at the frequencies shifted down from the frequencies of the pumping means by an amount determined by the Brillouin shift in the glass fibre, comprising Brillouin gain portions for obtaining gain in opposite directions at shifted frequencies, input/output couplers to provide input/output coupling and feedback through the ring cavity, an optical amplifier to compensate for the ring cavity losses, and a polarizing spun highly birefringent fibre (PSHBF) to prevent injection of the Brillouin pumping means into the optical amplifier.
The concept of the laser arrangement is as follows: First increase the gain of the optical amplifier to a level just below that required for the arrangement to operate as a laser. Elliptically polarized light from an external narrow linewidth laser is then injected into the predetermined portions of the ring cavity to stimulate Brillouin scattering in both clockwise and counterclockwise directions. In scalar-type backward stimulated Brillouin scattering, circularly or elliptically polarized Brillouin pump is reflected into the Stokes wave as if from a conventional mirror [B. Ya. Zeldovich, N. E. Pilipetsky, and V. V. Shkunov, in Principles of Phase Conjugation, Springer-Verlag, Berlin, 1985], ie. with the reversal of rotation sense and preservation of the orientation of the ellipse. With sufficient intensity of the injected Brillouin pump an elliptically polarized Brillouin signal is generated in the opposite direction and with the opposite rotation sense supported by the PSHBF at a frequency separated from the injected signal by the Stokes shift and it will eventually have sufficient gain to reach a lasing threshold by virtue of the combination of the Brillouin gain and the gain in the optical amplifier.
Preferable key features of the laser are that it does not need a low loss resonator, instead using an optical amplifier to equalise the resonator losses, which allows it to obtain high power output, and it does not require an optical isolator, instead using a PSHBF to isolate the Brillouin pump, thus allowing for the bidirectional operation. The PSHBF prevents both depletion of the optical amplifier gain by an injection locking to the Brillouin pumping means and provides feedback for the Brillouin signal.
In a preferred embodiment, the Brillouin gain portions are lengths of single mode optical fibre and the optical amplifier comprises a section of erbium doped fibre which is optically pumped through a wavelength division multiplexing (WDM) coupler. With sufficient pump power applied, the ring laser would operate as a normal erbium doped fibre ring laser. However, in this embodiment, laser action is originated from signals shifted down in frequency by an amount determined by the Brillouin shift in the glass fibre, generated from an external narrow linewidth laser source. The Brillouin pump signals injected in opposite directions are non-resonant in the laser cavity as they are blocked by the PSHBF in the ring and hence do not enter the EDF section of the ring. However the Brillouin signals generated in the opposite directions with respect to the corresponding Brillouin pumps are amplified by the amplifying section of the ring. The polarizing spun highly birefringent fibre prevents the ring from operating as an injection locked laser and sets the internal polarization state of the laser.
Such an arrangement has advantages in producing bidirectional high output power narrow linewidth laser sources for fibre sensors. The laser has only one elliptically polarized eigenstate in both clockwise and counter-clockwise directions. This makes the laser very attractive for current sensing using the Faraday effect. Additionally, the laser also has application in the gyroscope area. Elliptical polarization of the counter propagating laser eigenstates also ensures that spatial hole burning in the amplifying section of the laser is reduced as the standing wave pattern is close to a helicoidal shape and thus the light field intensity approaches uniformity along the doped fibre.
To measure nonreciprocal phaseshifts induced by the Faraday effect when an electric current is applied to a conducting coil wound around the PSHBF section of the laser, the frequency of the optical beat produced by the counter-propagating lasing modes can be analysed using a radio-frequency spectrum analyser (RFSA). For experimental convenience and to prevent possible lock-in of the counter-propagating lasing modes at low currents, the optical beat frequency in this embodiment is preferably biased using a frequency shifter by shifting the frequency of one of the Brillouin pumps, thus shifting the corresponding Brillouin gain maximum. Given the sweep time xcfx84 of the RFSA is much longer than the electric current waveform period T and the ratio xcfx84/T is known, the waveform can be reconstructed from the RFSA spectrum and the current can be measured.